This class unites substances that can be easily flared up by external sources of ignition. The fire can start as a result of friction, absorption of moisture, spontaneous chemical reactions, and also of heating. To this class belong pyrophoric substances, i.e., materials that ignite at contact with air even in small quantities.
The 4th class includes substances which emit the flammable gases at interaction with water. These gases are capable to form explosive mixes with air, and also the products containing such substances. It must be kept in mind that besides the flame the toxic products of burning can be dangerous too.
- Division 4.1: Flammable and self-decomposing solids
- Division 4.2: Substances liable to spontaneous combustion
- Division 4.3: Substances which, in contact with water, emit flammable gases